Heat-stabilized polychlorotrifluoroethylene



United States PatentfQ "ice HEAT-STABILIZED POLYCHLOROTRIFLUORO- ETHYLEN E Andrew T. Walter, South Charleston, W. Va., and Donald M. Young, New York, N.Y., assignors to Union Carbide Corporation, a corporation of New York No Drawing. Original application April 23, 1953, Serial No. 350,764, now Patent No. 2,784,170, dated May 5, 1957. Divided and this application March 8, 1956, Serial No. 570,220

Claims. (Cl. 260-459) by dispersing intimately therein certain inorganic compounds as for instance a salt of an oxyacid of nitrogen.

Polychlorotrifluoroethylene is a crystalline polymer melting at about 205 C. Its melting point is rather sharp, and similar in kind to that of polyethylene at 115 C. Its viscosity at temperatures above its melting point is high. For instance, the numerical expression of the melt-viscosity of a grade preferred for injectionmolding is approximately twenty-five times as great as that of the melt-viscosity of a commercial grade of polyethylene commonly used for extrusion and molding operations (sold under the trade name Bakelite polyethylene DYNH) at temperatures which are approximately 25 C. above their respective melting points. Also, the comparatively gradual decreasein viscosity with increase in temperature which is a characteristic of polyethylene is also observed with polychlorotrifluoroethylene. Were it possible to take melt-viscosity alone as the index of suitable performance of the resin for molding and extrusion operations the temperature that would be required to render a polychlorotrifluoroethylene resin equivalent to polyethylene at 150 C. would be about 360 C. (extrapolated). Furthermore, under high shearing forces, hot polychlorotrifluoroethylene has a tendency to crumble rather than flow, a tendency attributable to low intermolecular cohesion.

Because of these and other characteristics, the temperatures used in processing polychlorotrifluoroethylene, as by injection molding extrusion or the like are necessarily high. The operating temperature of the high order that is required falls within the temperature region in which breakdown in vacuum of all vinyl and diene polymers to gaseous materials within a short time has been observed. According to Research Paper RR-1938; National Bureau of Standards (L. A. Wall), complete decomposition of such a polymer within a short time is observed at 400 C. Mass spectrometer analyses, made inconnection with the present improvement, of gases evolved 2,879,257 Patented Mar. 24, 1059 at elevated temperatures from a polychlorotrifluoroethylene resin indicate that the degradation from the polymer to the chlorotrifluoroethylene monomer, although very small, is measurable at temperatures between 250 and 300 C. Above 300 C., the analyses also indicate, the rate of decomposition increases rapidly, and at about 375 C. the rate is so rapid that decomposition can be accomplished almost quantitatively in thirty minutes. Weight loss measurements, also made in connection with my improvement, have shown that at temperatures of 300 C. and 325 C. in open crucibles, fifty percent of 'the polychlorotrifluoroethylene resin is lost within fiftyseven and twelve hours, respectively; and that at temperatures of 340 to 350 C. the weight loss during the first hour of heating is extremely high. Although this loss in weight might be attributable in part to the evaporation of resin of low molecular weight, the seriousness of the depolymerization tendency is pointed up by the tests,

nevertheless. I

Melt-viscosity determinations on specimens which have been aged for varying lengths of time at temperatures ranging from 250 C. to 330 C. also provide a useful index of polymer breakdown. The method for determining melt-viscosity used herein is that described by G. J. Dienes and H. F. Klernm in the Journal of Applied Physics for June 1946 (vol. 17, No. 6, pp. 458-471). Upon being subjected to temperatures above 330 C. the melt viscosity, at 230 C., of even the most stable polymer samples falls below 0.05 megapoise Within a short time. A resin having a melt-viscosity at 230 C. of 0.05 megapoise or less is very weak and can easily be broken by hand, and at melt-viscosities of about 0.5 megapoise the increase in brittleness and the decrease in ultimate elongation is substantial. Typical data on the relation ship of physical properties and melt-viscosities at 230 C. are given in the following table:

TABLE I Comparison of m'elt-viscosities and physical properties of polychlorotrzfluoroethylene 230 C. Tensile Ultimate Brittle Sample No. Melt Strength, Elon- Temper- Viscosity, p.s.1. gation, ature, Megapolses percent O.

NorE.A possible non-uniform erystallinity in the molded test speel mens may be reflected in certain of the above values. For instance, Sample No. 1 (melt-viscosity, 44) seemingly should have shown a tensile strength and elongation as great as that obtained with Sample No. 2 (melt-viscosity, 16), and a brittle temperature no higher than that of Sample No. 2. Similarly, Sample No. 3 (melt-viscosity, 5.2), seemingly should have shown a tensile strength and elongation greater than that of Sample No. 4 (melt-viscosity, 1.9).

chlorotrifluoroethylene can be stabilized against color development and degradation of melt-viscosity at molding temperatures by intimately dispersing therein a small amount of a salt of an oxyacid of nitrogen, including the metal salts, ammonium salts and amine or carbamide salts. The salts include the nitrates and nitrites of these cations. The metal salts which are particularly useful as stabilizers include the salts of sodium and potassium in group 1 of the periodic table; the salts of magnesium, barium, calcium, strontium, cadmium, mercury and zinc in group 2; and the salts of aluminum in group 3, of zirconium and lead in group 4, and of bismuth in group 5. In addition to the foregoing metal salts which are preferred, the lithium, caesium, silver, gold, titanium, tin, vanadium and antimony salts of the oxyacids of nitrogen can be used with good results. Also, mixtures of the various metal salts, in particular a mixture of potassium nitrate and potassium nitrite, can be used.

From about 0.005 part to 2.5 parts of salt per 100 parts of polychlorotrifluoroethylene resin is useful for my improvement. Larger amounts may be used, if desired, but no added advantage results from the use of more than 2.5 parts per 100 parts of resin. An amount which is about 0.01 to 0.1 part per 100 parts of resin is preferred.

The salt can be dispersed in the resin by any suitable expedient. For instance, the resin and the salt can be dry-mixed or blended in powder form as by finely pulverizing the resin and the salt separately to pass a 100- mesh screen and then intimately mixing and blending them in a suitable mixer or blender. Good distribution of the salt stabilizer can be obtained also by first dissolving in a suitable solvent, for instance, a mixture of isopropanol, 80 parts, and water, 20 parts, and then tumbling the powdered resin with an amount of solution to give the desired salt content. When a salt solution is used, care should be taken to evaporate the solvent slowly to prevent migration of the salt in the final drying of the resin.

In addition to the foregoing method, the stabilizer may be incorporated in the resin by milling on a heated tworoll mill. In addition, the stabilizer may be added prior to, during, or subsequent to the polymerization of the chlorotrifiuoroethylene.

Polychlorotrifluoroethylene resins stabilized by a salt intimately dispersed therein in accordance with my discovery can be extruded, molded or the like at temperatures required for such operation without substantial deterioration or loss of essential properties. l

The improvement is further illustated by the following examples:

EXAMPLE 1 Various nitrate and nitrite salts were incorporated in polychlorotrifiuoroethylene resins as follows:

One part of a 1% solution of the stabilizer in a mixture of 80 parts of isopropanol and 20 parts of water was blended with 10 parts of dry resin and the solvent was removed by drying at 100 C. leaving the stabilizer intimately mixed with the powdered resin. The mixture of resin and stabilizer was then heated for 5 hours at 200 C. to remove all volatile impurities before testing.

The test procedure was as follows: I I

Ten grams of the mixture of resin and stabilizer were heated for one hour at 300 C. in an open glass tube and then molded at a temperature of 230 C. for five minutes to form disks 1.5 inches in diameter and /s inch in. thickness. Test disks having a diameter of five-eighths of an inch were then cut from the molded disks and a melt viscosity determined at 230 C. The melt viscosity of the stabilized resin was compared after heating with the melt viscosity of a control specimen (containing no stabilizer) and with the initial viscosity of the resin prior to heating at 300 C.

The results of the tests are given in the following table:

TABLE I Stabilization of polychlorotrifluoroethylene with nitrate and nitrate and nitrite salts 230 0. Melt Viscosity, Megapoises Color after Stabilizer, 0.1% Heating for After 1 hour at Before Heating 300 0. Heating for 1 hour at 300 G.

Control 9. 6 0. 6 amber. Aluminum nitrate. 9. 6 6. 1 Do. Ammonium nitrate 9. 6 1. 5 Do. Barium nitrate- 9. 6 1. 3 colorless Bismuth nitrate 9. 6 6. 1 Do. Cadmium nitrate 9. 6 6. 4 Do. Calcium nitrate 9. 6 1. 2 light amber. Lead nitrate 9. 6 2. 6 colorless. Magnesium nitrate 9. 6 3.0 amber. Mercuric nitrate- 9. 6 1. 3 colorless. Metbylurea nitrate 9. 6 5. 4 amber. Potassium nitrate- 9. 6 2.0 colorless. Sodium nitrate..-. 9. 6 1.0 Do. Sodium nitrite 9. 6 1. 0 Do. Strontium nitrate 9. 6 1. 3 Do. Potassium nitrate-Potassium nitrite. 9. 6 2.0 Do. Zinc nitrate 9. 6 4. 0 dark amber.

l3. 1 5. 6 amber.

13. 1 0. 3 black.

13. 1 1. 1 amber. Bismuth nitrate. 13. 1 3. 2 Do. Cadmium nitrate" 13. 1 4. 0 D0. Lead nitrate 13. 1 4. 0 Do. Magnesium nitrate 13. 1 5. 6 Do. Methylurea nitrate 13. 1 0. 3 Do. Potassium nitrate- 13. 1 3. 2 Do. Potassium nitrite l3. 1 1. 0 light amber. Sodium nitrate 13. 1 1. 2 colorless. Potassium nitrite-Potassium nitrate. 13.1 7. 0 o. Zinc nitrate 13. 1 1. 6 black.

EXAMPLE 2.

Various nitrate salts were incorporated in polychlorotritluoroethylene resins as follows:

Powdered polychlorotrifluoroethylene (20 grams) was thoroughly dried for five hours at 200 C. and then blended with 0.02 gram of the salt stabilizer. This mixture was fluxed on a two-roll mill at C. and further worked in by milling for five minutes at 185 C.

The test procedure was as follows:

Ten grams of the milled sheet were heated for one hour at 300 C. in an open glass tube and then molded at a temperature of 230 C. for five minutes to form disks 1.5 inches in diameter and /e inch in thickness. Test disks having a diameter of inch were then cut from the molded disks and melt viscosities determined at 230 C. The melt viscosity of the stabilized resin after heating was compared with the melt viscosity of similarly heated unstabilized resin and with the initial viscosity obtained on samples which were not heated at 300 C.

The results of the tests are given in the following table:

TABLE 11 Stabilization of polychlorotrifluoroethylene with nitrate stabilizers 230 0. Melt Viscos- EXAMPLE 3 Various nitrate and nitrite salts were tested as stabilizers for polychlorotrifluoroethylene, resins when subjected to both a heating and an extrusion operation.

The method of sample preparation and testing was as follows:

Five parts of a 1% solution of the salt in a mixture of 80 parts of isopropanol and 20 parts of water were blended with 95 parts of the dry, powdered resin and the blend of stabilizer and resin dried for two hours at 100 C. and then for five hours at 200 C. to remove all volatile impurities. A ten-gram sample was heated for one hour at 300 C. in a glass tube and then molded and tested for viscosity as previously described. Five pounds of each of the compositions were also extruded into a rod of approximately one-eighth inch diameter and of smooth surface from a 1% inch Modern Plastics Machinery Company extruder at a rate of about 8 to pounds per hour. The extruded rod was then molded and tested for viscosity as previously described.

The test results were as follows:

TABLE III Stabilization of p0lychlorotrifluoroethylene with nitrate and nitrite saltsExtrusion tests 230 0. Melt Viscosity, Mega- Color poises Initial -12.5

Stabilizer, 0.05%

After After Heating Ex- After Heating After Extrudior 1 trudfor 1 Hour at ing Hour at ing 300 0.

Control 0.7 1.3 Amber Dark amber. Bismuth nitrate 4.0 1.7 Colorless Colorless. Cadmium nitrate 6.0 1.6 Light amber- Lightamber. Lead nitrate 7.8 1.7 Colorless Grey. Magnesiumnitrate- 11.5 2.6 Light amber Light amber. Potassium nitrate.. 10.0 2.4 do Colorless. Potassium nitrite 9.0 3.7 do Do. Potassium nitrate 10.0 3.5 Colorless Do.

and potassium nitrite.

EXAMPLE 4 Potassium nitrite was tested as a stabilizer at different concentrations with several samples of polychlorotrifluoroethylene resins. The method of incorporating the stabilizers and the test methods were the same as described in Example 1.

The results of the tests were as follows:

TABLE IV Stabilization of polychlorotrifluoroethylene with potassium nitrite-Eflect of concentration 230 0. Melt Viscosity, Megapoises Color After Stabilizer After Heating ior 1 Before Heating Hour at 300 0. Heating for One Hour at Control No. 1 6. 1 1. 7 Light amber. Potassium nitrite, 0.02%.- 6. 1 6. 1 Colorless. Potassium nitrite, 0.047 6.1 4. 8 Do. Potassium nitrite, 0 067 6.1 3. 8 Do. Control No. 2 0. 6 Light amber. Potassium nitrite, 0.03%.- 20 3. 4 Colorless. Control No. 3 12. 5 0. 7 Light amber. Potassium nitrite, 0.05% 12. 5 9.0 Colorless.

EXAMPLE 5 Potassium nitrite in different concentrations was tested as a stabilizer for polychlorotrifiuoroethylene resins which were subjected to extrusion. The results of the tests are given in the table below.

TABLE V Stabilization of polychlorotrifluoroethylene with potassium nitrite 230 C. Melt Viscosity, Megapoises Sample Color After N o. Stabilizer Extruding Before After Extrud- Extruding ing 1 1 Control #1 5. 2 1. 6 Amber. 2 Ptatggsium nitrite, 5. 2 4. 4 Light amber. 3 Potassium nitrite, 5. 2 3. 5 Colorless.

0.04%. 4 Pcatggsium nitrite, 5. 2 2. 5 D0. 5 comm 11.1 2. 6 Amber. 6 Piatgzsium nitrite, 11. 1 8. 9 Light amber. 7 Potassium nitrite, 11.1 s. 4 Very light 0.04%. amber.

1 See Table V1 for extrusion conditions.

TABLE VI Extrusion conditions Sample No 1 2 3 4 5 6 7 Extrusion 'Iemp., F.:

Cylinders, avg Head As shown both in this example and in the preceding example, small amounts of the stabilizers are very effective, and the stabilizers are frequently more eiiective at lower concentrations than at higher concentrations.

This application is a division of our copending application Serial No. 350,764, filed April 23, 1953, Patent No. 2,784,170.

What is claimed is:

1. A polychlorotrifluoroethylene resin stabilized against color development and degradation of melt-viscosity at molding temperatures by the addition thereto after polymerization of about 0.01 to 2.5 parts per parts of resin, by weight, of a metal nitrate of the group consisting of alkaline-earth metal and alkali-metal nitrates.

2. A polychlorotrifluoroethylene resin stabilized against color development and degradation of melt-viscosity at molding temperatures by the addition thereto after polymerization of about 0.01 to 2.5 parts per 100 parts of resin, by weight, of an alkali-metal nitrate.

3. A polychlorotrifiuoroethylene resin stabilized against color development and degradation of melt-viscosity at molding temperatures by the addition thereto after polymerization of about 0.01 to 2.5 parts per 100 parts of resin, by weight, of an alkaline-earth metal nitrate.

4. A polychlorotrifluoroethylene resin stabilized against color development and degradation of melt-viscosity at molding temperatures by the addition thereto after polymerization of about 0.01 to 0.1 part per 100 parts of resin, of sodium nitrate.

5. A polychlorotrifluoroethylene resin stabilized against color development and degradation of melt-viscosity at molding temperatures by the addition thereto after polymerization of about 0.01 to 0.1 part per 100 parts of resin, of potassium nitrate.

References Cited in the file of this patent UNITED STATES PATENTS 2,491,444 Cox et a1. Dec. 13, 1949 2,557,474 Sanderson June 19, 1951 2,569,524 Hamilton Oct. 2, 1951 2,594,560 Howard Apr. 29, 1952 2,643,988 Walter June 30, 1953 

1. A POLYCHLOROTRIFUORETHYLENE RESIN STABLIZIED AGAINST COLOR DEVELOPMENT AND DEGRADING OF MELT-VISCOSITY AT MOLDING TEMPERATURES BY THE ADDITION THERETO AFTER POLYMERIZATION OF ABOUT 0.01 TO 2.5 PARTS PER 100 PARTS OF RESIN, BY WEIGHT TO A METAL NITRATE OF THE GROUP CONSISTING OF ALKALINE-EARTH METAL AND ALKALI-METAL NITRATES. 